This invention relates to air conditioners and more particularly to a clutch control for automobile type air conditioners.
In the past, automobile type air conditioner clutch controls have included mechanical devices that open and close circuits when the temperature deviates from a preset value or range of values to actuate the controls of an air conditioner to produce the required corrective action.
More recently, a temperature sensing circuit has been provided that includes a device that makes use of the change in resistivity of a semiconductor with change in temperature, such as, for example, a thermistor. A thermistor has a high negative temperature coefficient of resistance, so its resistance decreases as temperature rises. The thermistor has an external resistor network to which a detection circuit is connected to provide an output of a first state when the detected resistive value increases beyond a first point and an output of a second state when the detected resistive value decreases beyond a second point.
The detection circuit includes two comparators, a latch, and an internal resistive network; these elements are included in a commercially available timer, manufactured by Intersil Corporation and sold under the designation ICM7555. The comparators are connected to the external resistive circuit and to the internal resistive network. The comparator compares the resistance changes of the external resistance network with the control internal resistance network and sets the latch in the first state when the element resistance increases beyond a first transition point and in the second state when the element resistance decreases beyond a second transition point. The transition points are the result of hystersis of the external resistance circuit and thermistor. Those persons skilled in the art desiring a more detailed description of the thermistor controlled circuit are referred to U.S. Patent application, Ser. No. 601,270; filed: Apr. 17, 1984 for a "Temperature Sensing Circuit" (B4864).
The problem with the thermistor sensing circuit is that of transient motion. That is, high voltage pulses appear on the battery voltage to produce an irregular motion between the first and second states. Another problem relates to self-induced transients within the circuit. These transients destroy the credibility of the first and second states and if severe enough, the integrated circuits of which the device is made.
Another problem, attending the use of the thermistor sensing circuit in automobile air conditioner clutches, involves polarity. While some air conditioner clutches are designed for positive polarity (switching), others are designed for negative polarity (switching).